Pin get-pip.py to 3.2

As get-pip.py evolves based on pip 10, while we're still using
pip 9, changes in the way it can be used are causing problems.

For example, the ability to use --download is no longer there.

As such, let's pin to a known good version and leave it at that
until we no longer need to use this script. Version 3.2 maps to
pip 7.1.2 which fulfills our needs.

Change-Id: I49f6c9b238b42d4840c17af222e3bc82dfa6167f

multi-node-aio/bootstrap.sh

@@ -41,7 +41,7 @@ esac
 # Install pip
 if ! which pip &>/dev/null; then
     curl --silent --show-error --retry 5 \
-        https://bootstrap.pypa.io/get-pip.py | sudo python2.7
+        https://bootstrap.pypa.io/3.2/get-pip.py | sudo python2.7
 fi
 
 # Install bindep and tox

pxelinux-provisioning/bootstrap.sh

@@ -41,7 +41,7 @@ esac
 # Install pip
 if ! which pip &>/dev/null; then
     curl --silent --show-error --retry 5 \
-        https://bootstrap.pypa.io/get-pip.py | sudo python2.7
+        https://bootstrap.pypa.io/3.2/get-pip.py | sudo python2.7
 fi
 
 # Install bindep and tox